1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices with recessed oxide regions and more particularly to a method for applying a nitride mask directly on a substrate previously damaged by ion implantation and then annealed.
2. Description of the Prior Art
It is well known in the fabrication of semiconductors having recessed oxide regions which are delineated by masks composed for example of silicon nitride, that on growing the thick oxide region a thin protruding oxide also grows underneath the silicon nitride oxidation mask. This thin protrusion in what will be the gate region is termed "bird's beak" due to its beak-like cross-sectional profile and its presence is ascribed to lateral diffusion of oxygen underneath the thin silicon dioxide pad which is disposed between and separates the silicon nitride mask from the surface of the silicon substrate. The thin silicon dioxide pad is used because disposing the silicon nitride mask directly on the silicon substrate produces stress induced defects and dislocations in the silicon substrate which deleteriously affect device performance. Thus, the use of the silicon dioxide pad to prevent stress defects results in the undesired bird's beak condition.
Heretofore, attempts have been made to improve the fabrication process by continuing to use a separation pad and to minimize the resultant bird's beak condition.
For example, U.S. Pat. No. 3,900,350 issued Aug. 18, 1975 to Appels et al. and assigned to U.S. Philips Corporation teaches an approach to reducing the bird's beak condition by using a polycrystalline silicon pad under the oxidation mask instead of the usual silicon oxide. This patent also provides a substantial teaching of the stress defects that will occur when the silicon nitride oxidation mask is disposed directly on the silicon substrate.
U.S. Pat. No. 3,961,999 issued June 8, 1976 to Antipov and assigned to IBM Corporation also describes a method for minimizing the bird's beak problem. In this patent the usual silicon dioxide pad is located between the silicon substrate and the silicon nitride layer. The technique taught in this patent involves etching holes through the silicon dioxide pad, the holes correspond to the openings in the nitride mask to enable the undercutting and exposure of the underside of the silicon nitride layer at the periphery of the silicon dioxide layer openings.
The two aforementioned patents are typical of the prior art in that the approaches continue to employ a pad between the nitride mask and the silicon substrate and then attempt to minimize the resultant bird's beak problem that is caused thereby.
The method of the present invention is unique in that it eliminates the need for on the intermediate pad between the mask and the substrate which gave rise to the bird's beak problem in the first place. The method of the present invention permits the nitride mask to be disposed directly on the silicon substrate and eliminates the stress defects which have heretofore been caused by this arrangement. The method of its present invention includes the steps of initially damaging the surface of the silicon substrare by ion implantation to a controlled depth and then annealing to generate a dense dislocation network array which prevents the stress induced defect propagation from the masking layer.
It is known that ion implantation of silicon as employed in the present invention can be used to harden silicon. The publication of S. M. Hu in the IBM Technical Disclosure Bulletin, Vol. 19, No. 2, July 1976 entitled "Hardening Silicon Wafers by Ion Implantation" uses such techniques to reduce dislocations in thermally stressed silicon wafers.